A Templated Method to Bi2WO6 Hollow Microspheres and Their

Article pubs.acs.org/IC

A Templated Method to Bi2WO6 Hollow Microspheres and Their Conversion to Double-Shell Bi2O3/Bi2WO6 Hollow Microspheres with Improved Photocatalytic Performance Xiaona Li, Renkun Huang, Yanhua Hu, Yongjuan Chen, Wenjun Liu, Rusheng Yuan, and Zhaohui Li* Research Institute of Photocatalysis, Fujian Provincial Key Laboratory of Photocatalysis, State Key Laboratory Breeding Base, Fuzhou University, Fuzhou 350002, People's Republic of China S Supporting Information *

ABSTRACT: Bi2WO6 hollow microspheres with dimension of ca. 1.5 μm were synthesized via a hydrothermal method using polystyrene particles as the template. The as-prepared Bi2WO6 hollow microspheres can be further transformed to double-shell Bi2O3/Bi2WO6 hollow microspheres. The samples were fully characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, N2-sorption Brunauer− Emmett−Teller surface area, UV−vis diffuse-reflectance spectroscopy, and X-ray photoelectron spectroscopy. The as-formed double-shell Bi2O3/Bi2WO6 hollow microspheres exhibit enhanced photocatalytic activity due to the hollow nature and formation of the p−n junction between p-type Bi2O3 and n-type Bi2WO6. The study provides a general and effective method in the fabrication of composition and dimension-tunable composite hollow microspheres with sound heterojunctions that may show a variety of applications.

1. INTRODUCTION Hollow micro/nanosized spheres have shown wide applications as heterogeneous catalysts, drug-delivery agents, photonic materials, battery materials, and so on because of their unique chemical and physical properties compared to their solid counterparts.1,2 By far, the templated method is the most effective method in the preparation of hollow micro/nanosized spheres.3 Template-based syntheses of hollow spheres rely on formation of the desired sphere component on a sacrificial template followed by removal of the template.4 The advantage of template-based syntheses is that the shape and size of the cavity of the as-formed hollow spheres can be determined by the involved template.5 A variety of templates, such as polymer latex spheres,6 silica sol,7 gas bubbles, microemulsion droplets, etc., have been introduced in the preparation of hollow sphere materials.8,9 As a generally applied template, polystyrene (PS) particles exhibit many excellent characteristics such as simple preparation, ease for surface functionalization, and flexibility in size variation.10,11 The use of PS particles as templates in the preparation of a variety of hollow spheres has already been demonstrated.12,13 For example, conduction polymer polyaniline and polypyrrole hollow microspheres have been successfully prepared by oxidative polymerization of aniline or pyrrole on the surface of sulfonated PS followed by template removal by calcination.14,15 ZnO−TiO2 mixed metal oxide hierarchical hollow spheres have been fabricated by coating the functionalized PS template beads with successive layers of ZnO and TiO2 nanoparticles, followed by calcination of the resulting PS/ ZnO−TiO2 core−shell composite particles at elevated temperature.16 © 2012 American Chemical Society

Bi2WO6 is a promising visible-light-responsive photocatalyst for the degradation of organic pollutants and water splitting.17,18 Nanoparticles, nanoplates, as well as some complicated 3D hierarchical structures of Bi2WO6 including flowerlike microspheres,19 nestlike structure,20 hollow microspheres, and octahedron-like assemblies have all been reported.21,22 The photocatalytic performance of Bi2WO6 has been found to be greatly influenced by its morphology. Hollow spheres of Bi2WO6 show a superior photocatalytic performance because of a more efficient use of the light source via multiple reflections of light within the interior cavity.23 Combining two or more semiconductors with appropriate band positions to improve the photocatalytic performance of the semiconductors is an established idea because it can lead to an enhanced charge separation and interfacial charge-transfer efficiency.16,24 Especially, the fabrication of a p−n junction is believed to be the most effective because of the existence of an internal electric field.25 Actually, coupling Bi2WO6 to another semiconductor has also been tried previously, and a couple of composite photocatalysts comprised of n-type Bi2WO6 and p-type Bi2O3 have already been fabricated, which show an improved photocatalytic performance.26−29 However, to the best of our knowledge, the fabrication of hollow microspheres of a doubleshell Bi2O3/Bi2WO6 composite, which can make an effective and sound p−n junction between p-type Bi2O3 and n-type Bi2WO6, has never been reported previously. Received: February 29, 2012 Published: May 16, 2012 6245

dx.doi.org/10.1021/ic300454q | Inorg. Chem. 2012, 51, 6245−6250

Inorganic Chemistry

Article

Photocatalytic Test. The photocatalytic activity of the as-prepared samples was evaluated by the degradation of RhB under visible-light irradiation. A 500-W tungsten−halogen lamp was positioned inside a cylindrical Pyrex vessel and surrounded by a circulating water jacket (Pyrex) to cool the lamp. A cutoff filter was placed outside the Pyrex jacket to completely remove all wavelengths less than 420 nm to ensure irradiation with visible light only. A total of 80 mg of Bi2O3, Bi2WO6 (SSR), Bi2WO6, and Bi2O3/Bi2WO6 hollow microspheres was added to 80 mL of a RhB solution (10−5 mol L−1). Prior to irradiation, the suspensions were magnetically stirred in the dark for 4 h to ensure the establishment of an adsorption/desorption equilibrium. At given irradiation time intervals, 3 mL of the suspension was collected, centrifuged, and filtered through a Millipore filter to separate the photocatalyst. The degraded solutions were analyzed by a Varian Cary 500 Scan UV−vis spectrophotometer, and the absorption peak at 554 nm for RhB was monitored.

Herein, we report the preparation of hollow microspheres of Bi2WO6 via a PS template method and their transformation to the double-shell Bi2O3/Bi2WO6 hollow microspheres by a hydrothermal treatment. This method is expected to be applied in the fabrication of hollow microsphere composite materials with a sound heterojunction. The combination of the hollow structure and the existence of the p−n junction in the asprepared Bi2O3/Bi2WO6 composite render it with an improved photocatalytic performance under visible-light irradiation.

2. EXPERIMENTAL SECTION Materials. Styrene (ST; Fluka) and acetoacetoxyethyl methacrylate (AAEM; 97%, Aldrich) were distilled under N2. Sodium peroxydisulfate (SPDS; 97%), Bi(NO3)3·5H2O (99.9%), Na2WO4, and NaOH were used as received without further purification. Preparation of PS Particles. Functionalized PS particles with diameter of ca. 500 nm were prepared by the emulsion polymerization of ST and AAEM following a previously reported method.30 A doublenecked glass reactor equipped with a reflux condenser was purged with N2. Deionized water (170 mL) and appropriate amounts of ST and AAEM were added into the reactor, and the reaction mixture was stirred for 10 min at room temperature. Then the temperature was raised to 70 °C, and an aqueous solution of SPDS (0.30 g in 10 mL of deionized water) was added into the reaction mixture to initiate the polymerization process. The reaction was allowed to proceed at 70 °C for another 24 h, and PS particles were obtained as a stable dispersion in water with ca. 15% solid content. Preparation of Bi2WO6 Hollow Microspheres. Bi(NO3)3·5H2O (2.426 g, 5 mmol) was dissolved in 40 mL of deionized water, and the solution was stirred vigorously. A dispersion of the as-prepared PS particles (4 g) was added to the Bi(NO3)3 suspension, and the reaction was sonicated for 40 min. Subsequently, Na2WO4·2H2O (0.825 g) in 10 mL of deionized water was added slowly to the mixture, and the reaction mixture was sonicated for another 40 min. The resulting reaction mixture was transferred to a 100 mL Teflon-lined autoclave and was reacted at 180 °C for 48 h. After being cooled to room temperature, the sample was washed with deionized water several times and dried at 60 °C. Also, the product was calcined at 500 °C for 4 h to remove the PS template. The bulk Bi2WO6 (SSR) sample was prepared from Bi2O3 and WO3 via a conventional solid-state reaction according to the literature.31 Preparation of Bi2O3/Bi2WO6 Hollow Microspheres. A desired amount of Bi(NO3) 3·5H2O was dissolved in 20 mL of ethylene glycol (EG) and 40 mL of thanol, and the resulting solution was sonicated until it became clear. The as-obtained Bi2WO6 hollow spheres (1 g) were added to the solution and stirred. After that, the mixture was transferred to a Teflon-lined autoclave, sealed, and heated at 150 °C for 24 h. After being cooled to room temperature, the product was collected by filtration, washed with deionized water, and dried at 60 °C. Finally, the precursor was calcined at 450 °C for 3 h. Characterizations. Powder X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance X-ray diffractometer (Cu Kα1 irradiation, λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantum 2000 XPS system with a monochromatic Al Kα source and a charge neutralizer. All of the binding energies were referred to the C 1s peak at 284.8 eV of the surface adventitious carbon. The morphology of the sample was characterized by a field-emission scanning electron microscope (model JSM-6700F). The transmission electron microscopy (TEM) and highresolution TEM (HRTEM) images were measured by a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. The specific surface area of the samples was measured by N2 sorption at 77 K on an ASAP 2020 instrument and calculated by the Brunauer− Emmett−Teller (BET) method. UV−vis absorption spectra (UV−vis diffuse-reflectance spectrometry, DRS) of the powders were obtained for the dry-pressed disk samples using a UV−vis spectrophotometer (Cary 500 Scan spectrophotometer, Varian). BaSO4 was used as a reflectance standard in the UV−vis DRS experiment.

3. RESULTS AND DISCUSSION PS particles were prepared by the copolymerization of ST and AAEM according to a reported method.30 The scanning electron microscopy (SEM) image reveals that the as-prepared PS particles own smooth surfaces and the particles are nearly monodispersed in size with ca. 500 nm diameter (Figure 1a).

Figure 1. SEM images: (a) PS particles; (b) PS/Bi2WO6 composite microspheres; (c) Bi2WO6 hollow microspheres.

The added comonomer AAEM helps to stabilize the obtained colloidal system and renders the surface of the resulting PS particles with β-diketone functionality. For the formation of PS/Bi2WO6 core−shell composite particles, the first step is the adsorption of Bi3+ on the surface of PS, which can be facilely achieved because of the existence of the physical interaction between the surface β-diketone functionality and Bi3+ ions. In the next step, a Na2WO4 solution should be added slowly into the colloidal system to ensure its full reaction with the surfaceadsorbed Bi3+. An appropriate hydrothermal reaction temperature and reaction time is essential for the formation of welldefined core−shell PS/Bi2WO6 composite particles. Welldefined yarn-ball-like microspheres of PS/Bi2WO6 with dimension of about 1.5 μm can be obtained by heating the mixture at 180 °C for 48 h (Figure 1b). A lower hydrothermal temperature (160 °C) or a shorter reaction time (24 h) led to not well-formed microspheres, although the as-formed samples are also indexed to be orthorhombic Bi2WO6 (Figures S1 and S2 in the Supporting Information). To remove the interior PS core, the well-defined yarn-ball-like PS/Bi2WO6 microspheres were calcined at 500 °C and hollow yarn-ball-like microspheres with similar dimension were obtained, as evidenced from a broken microsphere shown in Figure 1c. Considering the size of the PS particles (500 nm), the resultant hollow microspheres should have a thick Bi2WO6 shell of about 500 nm. The thick Bi2WO6 shell is believed to provide enough mechanical support for these Bi2WO6 hollow microspheres after removal of the PS core. The XRD pattern of the sample shows 2θ peaks at 28.3°, 32.79°, 47.1°, 55.8°, 58.5°, 68.8°, 75.9°, and 78.4°, which correspond to the diffraction from the (131), (200), (202), 6246

dx.doi.org/10.1021/ic300454q | Inorg. Chem. 2012, 51, 6245−6250

Inorganic Chemistry

Article

characterized by TEM and HRTEM as well. In agreement with the SEM image, the TEM image of the composite shows that the microsphere has a dimension of about 1.5 μm, and the relatively pale center observed on the TEM image of a single microsphere further confirms that the as-prepared Bi2WO6/ Bi2O3 microsphere has a hollow center (Figure 3b). The dimension of the cavity is estimated to be about 500 nm, comparable to the size of the template PS particles. The HRTEM image of the composite shows two types of intimately contacted lattice fringes, confirming formation of the junction between Bi2WO6 and Bi2O3 (Figure 3c). The lattice spacing of 0.273 nm corresponds to the interplanar distance between adjacent (400) crystallographic planes of Bi2O3, while the fringe of d = 0.193 nm matches that of the (202) plane of Bi2WO6. XPS analyses were also carried out on the as-prepared Bi2O3/ Bi2WO6 composite. The XPS spectrum in the W 4f region shows a binding energy at 34.7 eV for W 4f7/2 and at 36.8 eV for W 4f5/2, suggesting that W exists in the chemical state of W6+ (Figure 4a).30 The high-resolution XPS spectra of the Bi 4f7/2 region can be deconvoluted into two peaks at around 158.8 and 160.1 eV and suggest that there are two types of Bi ions in the as-prepared Bi2WO6/Bi2O3 composite (Figure 4b). Considering that the binding energy of the Bi ion in pure Bi2WO6 exists in the range of 158−159 eV19,20,28 and that for Bi2O3 in the range of 159−160 eV,32 the peak at 158.8 eV can be ascribed to Bi3+ in Bi2WO6, while the other one at 160.1 eV originates from Bi3+ in Bi2O3. Because of the existence of several types of nonequivalent lattice O atoms, the XPS spectrum in the O 1s region is complicated and can be deconvoluted to three peaks at 533.8, 529.6, and 531.2 eV, which can be assigned to Bi−O in Bi2O3 and Bi−O and W−O in Bi2WO6 respectively (Figure 4c).20,28 The processes involved in the formation of Bi2WO6 hollow microspheres and their transformation to double-shell Bi2O3/ Bi2WO6 composite hollow microspheres are summarized in Scheme 1. This provides an effective and general method for the fabrication of composite hollow microspheres with an effective and sound heterojunction that may show a variety of applications. An added advantage of this method is that the dimension of the hollow center, the thickness of the shell, and the ratio of the two constituents can be facilely adjusted by deliberately controlling the reaction conditions. The as-prepared Bi2WO6 hollow microspheres show a BET surface area of 7.45 m2 g−1, which is about 30 times that prepared via the conventional solid-state reaction (0.28 m2 g−1). Bi2O3/Bi2WO6 hollow microspheres exhibit a BET specific surface area of 6.18 m2 g−1, comparable to that of the parent Bi2WO6 hollow microspheres. This indicates that the process during deposition of the Bi2O3 shell does not influence the BET specific surface area of Bi2WO6 hollow microspheres. The UV−vis DRS spectra of the as-prepared Bi2WO6 hollow microspheres, Bi2O3/Bi2WO6, pure Bi2O3, and a mechanical mixture of Bi2WO6 and Bi2O3 with 5 wt % Bi2O3 are shown in Figure 5. The absorption edges of pure Bi2O3 and Bi2WO6 are determined to be 443 and 425 nm, respectively, corresponding to band gaps of 2.8 and 2.9 eV. These values are comparable to those reported in the literature.28,33 The absorption edge of a mechanical mixture of Bi2WO6 and Bi2O3 is determined to be 434 nm, the value of which lies between those of pure Bi2O3 and Bi2WO6. However, to our surprise, the Bi2O3/Bi2WO6 composite hollow microspheres show an additional transition, with the absorption edge extending to 495 nm. We believe that a partial interfacial charge transfer between p-type Bi2O3 and n-

(331), (262), (400), (193), and (402) crystallographic planes of orthorhombic Bi2WO6 (JCPDS, No. 39-0256). This indicates that the shell of the microspheres is composed of pure orthorhombic Bi2WO6 (Figure 2a).

Figure 2. XRD patterns: (a) Bi2WO6 hollow microspheres; (b) Bi2O3/ Bi2WO6 hollow microspheres.

To fabricate a Bi2O3/Bi2WO6 composite hollow microsphere, a thin layer of Bi2O3 was deposited over the as-formed Bi2WO6 hollow microspheres using Bi(NO3)3 as the Bi source and a mixture of EG and ethanol as the solvent. Strong coordinating solvent EG was chosen because it can react with the surface OH groups on the as-prepared Bi2WO6 microspheres by forming hydrogen bonds, and then the surface-adsorbed EG can further coordinate with Bi3+ to form a stable intermediate Bi 2 (OCH 2 CH 2 O) 3 . Hydrolysis of the intermediate Bi2(OCH2CH2O)3 followed by calcination at 450 °C led to the formation of Bi2O3 nanoparticles. Such a process can ensure the formation of a thin shell of Bi2O3 on the surface of Bi2WO6 hollow microspheres to make an effective and sound Bi2O3/ Bi2WO6 p−n junction. The successful deposition of Bi2O3 on the surface of Bi2WO6 hollow microspheres was confirmed by the XRD pattern. In addition to the diffraction peaks assigned to orthorhombic Bi2WO6, the XRD pattern of the as-formed Bi2O3/Bi2WO6 composite shows 2θ peaks at 28.0°, 31.8°, 32.7°, 46.3°, 54.4°, and 74.7°, which correspond to (221), (002), (400), (402), (223), and (750) planes of tetragonal Bi2O3 (JCPDS, No. 01-074-1374; Figure 2b). The composite maintains its original morphology as microspheres except for the much smoother surface compared to the parent Bi2WO6 hollow microspheres (Figure 3a). The size of the composite

Figure 3. Images of Bi2O3/Bi2WO6 hollow microspheres: (a) SEM; (b) TEM; (c) HRTEM.

microspheres does not change obviously, indicating that the asformed Bi2O3 shell is thin compared to the original Bi2WO6 shell. Theoretically, it is possible to tune the thickness of the Bi2O3 shell by changing the conditions during the Bi2O3 deposition process. To further obtain information about the structure of the sample, the as-prepared composite was 6247

dx.doi.org/10.1021/ic300454q | Inorg. Chem. 2012, 51, 6245−6250

Inorganic Chemistry

Article

Figure 4. High-resolution XPS spectra of Bi2O3/Bi2WO6 hollow microspheres: (a) W 4f; (b) O 1s; (c) Bi 4f.

Scheme 1. Processes Involved in the Formation of Bi2WO6 Hollow Microspheres and Their Transformation to DoubleShell Bi2O3/Bi2WO6 Composite Hollow Microspheres

Figure 6. Temporal changes of the RhB concentration as monitored by the UV−vis absorption spectra at 554 nm under visible-light irradiation: (a) no catalyst; (b) Bi2O3; (c) Bi2WO6 (SSR); (d) Bi2WO6 hollow microspheres; (e) Bi2O3/Bi2WO6 hollow microspheres.

Scheme 2. Schematic Diagram of Charge Transfer between n-Type Bi2WO6 and p-Type Bi2O3: (A) before Contact; (B) after Formation of the p−n Junction

Figure 5. UV−vis DRS spectra: (a) Bi2O3; (b) Bi2O3/Bi2WO6 hollow microspheres; (c) mechanical mixture of Bi2WO6 and Bi2O3; (d) Bi2WO6 hollow microspheres.

type Bi2WO6 is probably responsible for this absorption because such a transition cannot be assigned to either Bi2O3 or Bi2WO6. The photocatalytic performance of the samples was evaluated by decomposing a RhB aqueous solution under visible-light irradiation, and the results are shown in Figure 6. It is observed that Bi2O3 was almost photocatalytic inactive and only 8% of RhB was converted in 3 h over Bi2WO6 prepared via a solidstate reaction. On the contrary, 68% of RhB was converted over irradiated Bi2WO6 hollow microspheres in 3 h. Moreover, the incorporation of p-type Bi2O3 into n-type Bi2WO6 led to an obviously enhanced photocatalytic activity, and an almost complete degradation of RhB occurred within 3 h of irradiation. The improved performance observed over the Bi2O3/Bi2WO6 composite compared to Bi2WO6 hollow microspheres can be ascribed to formation of the p−n junction. Scheme 2a shows the energy-band schematic diagram for p-type Bi2O3 (ECB = 0.33 V and EVB = 3.13 V versus NHE) and n-type Bi2WO6 (ECB

= 0.36 V and EVB = 3.26 V versus NHE). When p-type Bi2O3 and n-type Bi2WO6 are contacted, the Fermi level of p-type Bi2O3 moves up, while in the meantime, that of n-type Bi2WO6 moves down until the equilibrium state is formed. Consistent with the moving of the Fermi level, the whole energy band of ptype Bi2O3 is raised up, while that of n-type Bi2WO6 is descended (Scheme 2b). An inner electric field from n-type Bi2WO6 to p-type Bi2O3 is thus established. Under visible-light irradiation, both Bi2O3 and Bi2WO6 can be excited to generate electron−hole pairs.34 According to the energy-band schematic diagram shown in Scheme 2b, the photogenerated electrons on the conduction band of p-type Bi2O3 can transfer to that of ntype Bi2WO6, while simultaneously photogenerated holes can migrate from the valence band of n-type Bi2WO6 to that of ptype Bi2O3. Such migrations of the photogenerated carriers can be promoted by the internally formed electric field. Thus, the 6248

dx.doi.org/10.1021/ic300454q | Inorg. Chem. 2012, 51, 6245−6250

Inorganic Chemistry

Article

(3) Zhang, D. F.; Zhang, H.; Shang, Y.; Guo, L. Cryst. Growth Des. 2011, 11, 3748−3753. (4) Strandwitz, N. C.; Stucky, G. D. Chem. Mater. 2009, 21, 4577− 4582. (5) Jia, G.; You, H. P.; Song, Y. H.; Huang, Y. J.; Yang, M.; Zhang, H. J. Inorg. Chem. 2010, 49, 7721−7725. (6) Su, Y.; Yan, R.; Dan, M. H.; Xu, J. X.; Wang, D.; Zhang, W. Q.; Liu, S. G. Langmuir 2011, 27, 8983−8989. (7) Tong, S. T.; Mao, L.; Zhang, X. H.; Jia, C. Q. Ind. Eng. Chem. Res. 2011, 50, 13825−13830. (8) Mazur, M. J. Phys. Chem. C 2008, 112, 13528−13534. (9) Carroll, N. J.; Pylypenko, S.; Atanassov, P. B.; Petsev, D. N. Langmuir 2009, 25, 13540−13544. (10) Shi, W.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N. Langmuir 2005, 21, 1610−1617. (11) Li, H. Q.; Ha, C. S.; Kim, I. Langmuir 2008, 24, 10552−10556. (12) Duan, G. T.; Lv, F. J.; Cai, W. P.; Luo, Y. Y.; Li, Y.; Liu, G. Q. Langmuir 2010, 26, 6295−6302. (13) Deng, Z. W.; Chen, M.; Gu, G. X.; Wu, L. M. J. Phys. Chem. B 2008, 112, 16−22. (14) Park, M. K.; Onishi, K.; Locklin, J.; Caruso, F.; Advincula, R. C. Langmuir 2003, 19, 8550−8554. (15) Fujii, S.; Matsuzawa, S.; Nakamura, Y.; Ohtaka, A.; Teratani, T.; Akamatsu, K.; Tsuruoka, T.; Nawafune, H. Langmuir 2010, 26, 6230− 6239. (16) Agrawal, M.; Gupta, S.; Pich, A.; Zafeiropoulos, N. E.; Stamm, M. Chem. Mater. 2009, 21, 5343−5348. (17) Shang, M.; Wang, W. Z.; Sun, S. M.; Zhou, L.; Zhang, L. J. Phys. Chem. C 2008, 112, 10407−10411. (18) Shi, R.; Huang, G.; Lin, J.; Zhu, Y. F. J. Phys. Chem. C 2009, 113, 19633−19638. (19) Liu, S. W.; Yu, J. G. J. Solid State Chem. 2008, 181, 1048−1055. (20) Wu, J.; Duan, F.; Zheng, Y.; Xie, Y. J. Phys. Chem. C 2007, 111, 12866−12871. (21) Li, Y. Y.; Liu, J. P.; Huang, X. T.; Li, G. Y. Cryst. Growth Des. 2007, 7, 1350−1355. (22) Li, Y. Y.; Liu, J. P.; Huang, X. T. Nano Lett. 2008, 3, 365−371. (23) Ma, D.; Huang, S. M.; Chen, W. X.; Hu, S. W.; Shi, F. F.; Fan, K. J. Phys. Chem. C 2009, 113, 4369−4374. (24) Lv, K.; Li, J.; Qing, X. X.; Li, W. Z.; Chen, Q. Y. J. Hazard. Mater. 2011, 189, 329−335. (25) Nikoobakht, B.; Bonevich, J.; Herzing, A. J. Phys. Chem. C 2011, 115, 9961−9969. (26) Shang, M.; Wang, W. Z.; Zhang, L.; Sun, S. M.; Wang, L.; Zhou, L. J. Phys. Chem. C 2009, 113, 14727−14731. (27) Xiao, Q.; Zhang, J.; Xiao, C.; Tan, X. K. Catal. Commun. 2008, 9, 1247−1253. (28) Gui, M. S.; Zhang, W. D.; Su, Q. X.; Chen, C. H. J. Solid State Chem. 2011, 184, 1977−1982. (29) Ge, M.; Li, Y. F.; Liu, L.; Zhou, Z.; Chen, W. J. Phys. Chem. C 2011, 115, 5220−5225. (30) Agrawal, M.; Pich, A.; Zafeiropoulos, N. E.; Gupta, S.; Pionteck, J.; Simon, F.; Stamm, M. Chem. Mater. 2007, 19, 1845−1852. (31) Zhang, S. C.; Zhang, C.; Man, Y.; Zhu, Y. F. J. Solid State Chem. 2006, 179, 62−69. (32) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Chanhassen, MN, 1995. (33) Bian, Z. F.; Zhu, J.; Wang, S. H.; Cao, Y.; Qian, X. F.; Li, H. X. J. Phys. Chem. C 2008, 112, 6258−6262. (34) Dai, G. P.; Yu, J. G.; Liu, G. J. Phys. Chem. C 2011, 115, 7339− 7346. (35) Guan, M. L.; Ma, D. K.; Hu, S. W.; Chen, Y. J.; Huang, S. M. Inorg. Chem. 2011, 50, 800−805. (36) Mandal, S.; Sathish, M.; Saravanan, G.; Datta, K. K. R.; Ji, Q. M.; Hill, J. P.; Abe, H.; Honma, I.; Ariga, K. J. Am. Chem. Soc. 2010, 132, 14415−14417. (37) Yu, J. G.; Yu, X. X. Environ. Sci. Technol. 2008, 42, 4902−4907.

photogenerated electrons and holes can be separated effectively because of formation of the p−n junction between p-type Bi2O3 and n-type Bi2WO6 interfaces. The separated photogenerated electrons and holes are then free to initiate reactions with the reactants adsorbed on the photocatalyst surface, leading to an enhanced photocatalytic activity.35 The hollow nature of the as-prepared Bi2O3/Bi2WO6 hollow microspheres is also responsible for its superior photocatalytic performance. The improvement of the catalytic activity due to the existence of the hollow morphology has been previously reported.36 Especially in photocatalysis, such a hollow structure can also allow multiple reflections of UV−vis light within the hollow center, which can make more efficient use of the light source.37 Moreover, photocatalysts in microsphere morphology can be more readily separated from the slurry system by filtration after the photocatalytic reaction and reused than the nanosized samples because of their large weight, weak Brownian motion, and good mobility.38 All of these make the photocatalysts with hollow microsphere morphology appealing photocatalysts in the aqueous photocatalytic reaction.

4. CONCLUSION In summary, a templated method has been used to synthesize Bi2WO6 hollow microspheres, which can be further converted to double-shell Bi2O3/Bi2WO6 hollow microspheres. The asformed Bi2O3/Bi2WO6 hollow microspheres exhibit obviously enhanced photocatalytic activity because of its hollow center and the existence of the p−n junction between p-type Bi2O3 and n-type Bi2WO6. The study provides a general and effective method in the fabrication of composition and dimensiontunable composite hollow microspheres with a sound heterojunction that may show a variety of applications.

■

ASSOCIATED CONTENT

S Supporting Information *

SEM images and XRD patterns of PS/Bi2WO6 composite microspheres synthesized under different reaction conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-591-83779260. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (Grants 20977016 and U1033603), National Basic Research Program of China (973 Program 2011CB612314), and Program for Changjiang Scholars and Innovative Research Team in University (Program PCSIRT0818). The Award Program for Minjiang Scholar Professorship and the NSF for Fujian Province for Distinguished Young Investigator Grant 2009J06004 to Z.L. are also acknowledged.

■

REFERENCES

(1) Li, X. X.; Xiong, Y. J.; Li, Z. Q.; Xie, Y. Inorg. Chem. 2006, 45, 3493−3495. (2) Zhu, H. T.; Wang, J. X.; Wu, D. X. Inorg. Chem. 2009, 48, 7099− 7104. 6249

dx.doi.org/10.1021/ic300454q | Inorg. Chem. 2012, 51, 6245−6250

Inorganic Chemistry

Article

(38) Yu, J. G.; Yu, X. X.; Huang, B. B.; Zhang, X. Y.; Dai, Y. Cryst. Growth Des. 2009, 9, 1474−1480.

6250

dx.doi.org/10.1021/ic300454q | Inorg. Chem. 2012, 51, 6245−6250